Technical Field
The present application relates to a mail separating device. More specifically it relates to a device incorporated into a feeder of a mail processing machine such as for instance a franking or sorting machine. The device can be used to separate letters or flats of various thickness and size, which may form homogeneous or heterogeneous batches.
Description of the Related Art
In automatic franking machines, mail is generally loaded as a batch, or stack, from which the envelopes must be separated. This process, also referred as singulation, is necessary to detect the leading edge of each envelope in order to print the franking mark at the right position. Moreover a certain gap shall be left between successive envelopes so that the machine has enough time to calculate the postage and print the franking mark before another envelope is detected. Advanced franking machines are also capable of measuring and weighing each envelope to calculate the appropriate postage rate. It is easily understood that double or multiple feed may cause either a jam or a malfunction of the franking process.
The problem is further complicated in a mix mail environment because of the various thickness, size, weight and materials of the envelopes. Indeed, mail batches can be either homogeneous or heterogeneous. In the last case envelope properties will not only vary from one batch to another, but also within the same batch. In the case of homogeneous batches the operator may adjust some parameters of the franking machine in order to cope with the properties of the batch. This is not possible in the case of a heterogeneous batch. Such adjustments are anyway not desirable due to the risk of errors and required productivity. The franking machine must therefore accept a wide range of mixed envelopes without settings.
Additionally high speed machines are designed so that mail can be reloaded on the fly, which brings other limitations. For instance when the batch is disposed vertically—with envelopes on top of each other—it is required to put the largest envelopes at the bottom and the smallest at the top so that the stack remains stable. The operator must wait until the small envelopes of the previous batch have been separated before reloading large envelopes.
Although other principles are possible, most franking machines use differential friction to separate envelopes. At least one driving belt or roller is arranged at the bottom of the stack to pull the first envelope away while the following ones are held by a retaining mechanism. This is possible because the friction between the driving belt or roller and the first envelope is higher than between the first and second envelopes. The retaining mechanism is either a set of friction pads or a belt moving in the direction opposite to the driving belt or roller. This mechanism is generally arranged to form a slope or funnel in order to stagger the envelopes.
Of course the friction force generated by the retaining mechanism is lower than the force generated by the driving belt or roller, so that a single envelope can pass through. As envelopes are separated, the stack becomes smaller and the pressure on the first envelopes decreases until there is only one envelope left. The last two envelopes must be separated whatever their size, weight or thickness.
Thus, for ensuring a perfect separation, many parameters can be adjusted: friction coefficient of the driving and retaining belts, rollers or pads, number disposition, angle and pressure of the retaining belt or pads, number, disposition, speed and acceleration of the driving and retaining belts or rollers. However it is not always possible to find a compromise which ensures separation of letters in all mail batch configurations, notably with the requirement of no operator settings. In particular, parameter values ensuring good separation of the bottommost envelope of a large and heavy batch may not be optimal when the batch is down to a few letters.
Another aspect to take into account is the dispersion of friction coefficient between letters. The friction coefficient is not equal between all envelopes, even in homogeneous batches. When the stack is accelerated by the feed rollers, it tends to split where the friction is the lowest and form blocks of envelopes temporarily bound together. Once formed, these blocks can be difficult to break down into single envelopes. It has been observed that the friction between envelopes tends to decrease with repeated sliding. The decrease of static friction can be of 20% after the third sliding. In other words, once a block is formed, its outermost envelopes tend to slide better against adjacent blocks.
Several solutions exist to automatically adjust some parameter values in relation to the mail batch configuration, and notably to the weight, or height, of the batch.
EP2199237 discloses a method of controlling and varying the drive speed of a motor driven conveyor in function of the weight of a stack of mailpieces standing on a receiving deck, in order to extract sail mailpieces one-by-one. It is known from academic studies that the friction coefficient for paper on rubber i) decreases with the applied load and ii) increases with differential speed. In a preferred embodiment, the weight of the stack is obtained from a weighing unit that carries the mailpiece receiving deck. The weight is constantly monitored and the drive speed decreases linearly as the stack decreases. The drive speed is thus at a maximum when the stack of mailpieces is the heaviest. Though the application of at higher drive speed for heavy stacks compensates the lowest friction coefficient to some extent, it only works insofar that the conveyor starts sliding on the bottommost envelope. This can result in soiling or even damaging the thinnest envelopes. Moreover the design of the weighing unit is rather complex and the weight signal is not accurate enough for other purposes, like for instance calculating the postage amount for each extracted envelope.
EP2586737 discloses an apparatus for processing items having a feed station with an ejection roller operable to remove an item from a stack of mail, and comprising a pressure element mounted to pivot and configured to exert a pressure force on said stack that causes a bottommost item of mail in said stack to be pressed against said ejection roller. The pressure element is configured to change said pressure force in dependence on a loss of weight of said stack due to removal of said bottommost item of mail. As the stack of mail decreases, the load applied by the pressure element increases and compensates the loss of weight. This is particularly useful when the stack is down to a few letters. It is indeed a problem for many separation devices to extract the very last letters because the small remaining weight does not generate enough friction against the ejection roller. However, the pressure element applies an extra load when the stack is the heaviest, thus increasing the risk of double-feed and limiting the capacity of the feeder. The pressure element represents an additional cost and is not convenient when the feeder has to be reloaded on the fly.
EP2292540 discloses a device having a clutch unit for actuating drive rollers that support a stack of mailpieces. An optical sensor detects the passage of the mailpieces entering into a separation zone that individually selects the mailpieces. A control unit controls repeated activation/deactivation of the clutch unit as long as the sensor is not activated, to cause a jerky motion of the stack for facilitating mutual separation of the mailpieces. The rapid succession of horizontal movements has indeed the effect of breaking down blocks of letters that are bound together and might not be separated properly. Though this method is particularly useful for heavy stacks it does not provide any advantage when the stack is down to a few letters, as it does not generate more friction against the drive rollers. When the stack is small the friction force may not be enough to drive the very last letters through the separation mechanism, and repeated activation/deactivation of the clutch unit may not generate any movement but only result in soiling or even damaging the thinnest envelopes.